WASHINGTON — Young voters who had been enthralled by Barack Obama’s “Yes, we can” message are now saying “Maybe not” — and are backing away from the president in a worrisome new poll for the White House.

Obama is losing in a match-up against a generic Republican challenger by 37 percent to 34 percent among voters in the 18-34 age group, according to a stunning Quinnipiac University poll released yesterday.

In March, voters in this group approved of Obama by 54 percent to 37 percent.

“The youngest age group may be the most impatient and the most easily disillusioned among all age groups,” said Molly Andolina, a youth-vote expert and DePaul University political-science professor.

For many young voters in 2008, “it was the first time they’d been really been involved, really paying attention. This is someone telling them, ‘Yes, you can,’ ” she said.

Two years later, with a prolonged Gulf oil spill, “watching how slow it is to respond may be a little disillusioning,” she added.

Obama won an astounding 66 percent of the vote among the under-30 crowd, according to exit polls tracked by the Pew Research Center, the biggest winning percentage for a candidate since Richard Nixon in 1972.